Just a guess.
I was setting transport header in in-correct place. I had a "publish" action in the proxy and I was setting transport header just before the publish action. I moved the "transport header setting" to be done inside the "publish" action/task . Now it works as expected.
Taken from https://community.oracle.com/thread/2155298?tstart=165, link most likely will be dead soon enough - Oracle community ...